Variable-length information recording with reference marks for time reference during information reading

Abstract

The information is recorded in the form of variable-length alterations (l 0 , l 0 +NΔl), for example in the form of localized ablations of a thermosensitive layer of the medium. This type of coding of the information is sensitive to imprecisions in the form factor which are due to the non-linearity inherent in the recording method used. The method according to the invention consists in recording marks (M 1 , M 2 ) serving as length reference, from place to place. These marks (M 1 , M 2 ) are recorded at the same time and in the same conditions as the information proper (d 1 , d 2 , d 3 ). During the read process, they serve as length references for the associated information area, and after conversion into electrical signals as time reference.

I claim: 
     
       1. A method for recording and reading information on a movable media having a sensitive layer and being driven at a uniform speed, said method comprising the steps of: recording said information during a writing phase including a first step of forming first variable-length alterations in said sensitive layer separated from one another by unaltered regions, by converting variable duration pulsed information signals wherein transitions between said alterations and said unaltered regions represent said information and wherein said writing phase further includes a second step of forming a series of sequences of length-reference second alterations in said sensitive layer, each of said second alterations being of different length and each sequence of said series of sequences including at least two alterations and wherein each sequence of said series of second alteration is associated with a predetermined area in which said first variable-length alterations are formed and wherein each of said sequences of alterations of said series forms a length reference for said predetermined area;   reading said information in a read phase including a first step of reading said length-reference second alterations and converting said length-reference second alterations into duration-reference pulsed signals, for a respective associated with predetermined area and said read phase further includes a second step of reading said first variable-length alterations representing information formed in said associated predetermined area and converting said first variable-length alterations into pulsed signals and a third step of reproducing said recorded information by comparison with said duration-reference pulsed signals.   
     
     
       2. Method according to claim 1 wherein the pulsed information signals are modulation signals digitally coded in duration, by jumps of time increments, the said first variable length-reference alternations are distributed over the medium in pairs and formed by conversion of pulsed signals of a first duration and of a second duration, these durations differing by at least one time increment. 
     
     
       3. Method according to claim 2 wherein said first duration is chosen such that the length-reference alteration obtained with the aid of the signal of this duration has a minimal length, this length being in a one-to-one relation with a minimal duration of the modulation signals, and the said second duration is chosen such that the alteration obtained with the aid of the signal of this duration has the said minimal length, to which is added a defined number of increments N of length, N being equal to the maximum number of time increments of the modulation signals; and in that the step of reproducing the recorded information comprises the determining of the durations of the converted signals corresponding to the alterations serving a length reference, and the determination, on the basis of these durations, of parameters in one-to-one relation with the said minimal duration and the time increment. 
     
     
       4. Method according to claim 3 wherein the reproduction step comprises the calculation of the duration of one time increment by subtraction from the duration of the signal resulting from the conversion of the length-reference alteration of maximal length, of the duration of the signal resulting from the conversion of the length-reference alteration, of minimal length, followed by the division by the said number N, this time increment serving as a reference for the said associated predetermined area. 
     
     
       5. Method according to any one of claims 2 to 4 and 1, wherein fluctuations of the parameters associated with the reference marks are stored in memory and in that comparisons are made with reference values in order to ignore the marks of which the said associated parameters are outside a predetermined range. 
     
     
       6. Method according to any one of claims 2 to 4 and 1, wherein, the said medium being equipped with a superficial thermosensitive layer, the alterations consist of a localized ablation of the said layer by irradiation of the latter with the aid of a laser beam. 
     
     
       7. Method according to any one of claims 2 to 4 and 1, wherein said medium including, alternately, distributed along tracks, areas of a predetermined first type in which said first alterations are formed during a preliminary, pre-recording phase, and areas of a second type intended for recording the said information, the sequences of length-reference alterations are formed in a defined region of the areas of the first type and are associated with a contiguous area of the second type. 
     
     
       8. Method according to claim 7 wherein said method comprises, in a preliminary phase, a step of prerecording information used to compensate, upon recording or upon reading information, for the offset between the actual site of each said first alteration of the said layer on the medium and a desired site. 
     
     
       9. Method according to claim 8, wherein said prerecorded information comprise data making it possible to calculate said offset as a function of parameters related to at least one alteration previously formed on the medium. 
     
     
       10. Method according to claim 9, wherein the parameters relating to the previously formed said at lease one alteration are its length and the distance separating said at least one alteration from a subsequent alteration to be formed. 
     
     
       11. Method according to any one of claims 2 to 4 and 1, wherein said medium consists of a disk. 
     
     
       12. Device according to claim 1, wherein further comprises a logic circuit enabling determination of the number of increments only during the periods separating two detections of said information alterations representing information. 
     
     
       13. Device according to claim 1, wherein said means for converting the lengths of the first variable-length alternations into numbers comprises a digital counter receiving and counting clock signals of much higher frequency than the frequency of occurrence of alteration detection, a logic circuit allowing counting only during the periods of detection of the said alterations and means for resetting the counter to an initial state after the end of the detection of the said alterations. 
     
     
       14. A device for recording/reading information along tracks of a moveable medium being driven at a uniform speed, said device comprising: means for detecting length-reference alterations in a sensitive layer of said movable medium and information representing alterations in said layer representing recorded information, and for converting said length-reference alterations and said information alterations into electrical signals;   means for converting the length of said detected length-reference alterations and said information alterations into numbers proportional to said length;   means for storing in memory at least said numbers corresponding to the detection of said length-reference alterations; and   calculating means for determining a number of increments associated with each said information alteration detected and converted into one of said electrical signals, on the basis of said numbers and of a maximum number of increments (N).   
     
     
       15. A method for recording and reading information on a movable media having a reference surface and being driven at a uniform speed, said method comprising the steps of: recording said information during a writing phase including a first step of forming first variable-length deviations from said reference surface by converting variable duration pulsed information signals wherein transitions from said deviations to flat surface areas between said deviations represent said information and wherein said writing phase further includes a second step of forming a series of sequences of length-reference second deviations from said reference surface, each of said second deviations being of different length and each sequence of said series of sequences including at least two deviations and wherein each sequence of said series of second deviations is associated with predetermined area in which said first variable-length deviations are formed and wherein each of said sequences of deviations are formed and wherein each of said sequences of deviations of said series forms a length reference for said predetermined area;   reading said information in a read phase including a first step of reading said length-reference second deviations and converting said length-reference second deviations into duration-reference pulsed signals, for a respective associated said predetermined area and said read phase further includes a second step of reading said first variable-length deviations representing information formed in said associated predetermined area and converting said first variable-length deviations into pulsed signals and a third step of reproducing said recording information by comparison with said duration-reference pulsed signals.